In late spring, the San Juan charged like a desert bighorn ram in the rut. It was strewn with cottonwood seeds: The puffy globs drifted on the khaki surface like a monsoon over a desert escarpment.

Drifting down the Upper San Juan River in southern Utah, life slowed to the speed of the river and the rhythm of nature. Over the next four days, sheer cliffs rose above the water; great blue ...